Inkjet printing mechanisms fire drops of ink onto a print medium to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. An inkjet printing apparatus may include a printhead having a plurality of independently addressable firing units. Each firing unit may include a fluid chamber connected to a fluid source and to a fluid outlet nozzle. A transducer within the fluid chamber provides the energy for firing fluid drops from the nozzles. In thermal inkjet printers, the transducers are thin-film resistors that generate sufficient heat during application of a voltage pulse to vaporize a quantity of fluid. This vaporization is sufficient to fire a fluid drop.
It is known to control drop quantity for inkjet printing of colored inks, for example by comparing the color of a calibration pattern printed on a print medium with a desired color output. This comparison can be useful to compensate for deterioration of a printhead over time, for example due to kogation. However, such techniques cannot be used for a colorless fluid, such as a pretreatment fluid for improving the fixing of a colored ink to the print medium, to reduce image quality defects.